Dithionite (hydrosulfite) is an agent generally used in bleaching of different products, such as pulp and textiles, and in other subjects. Several methods for preparing dithionite are generally known in the art. Because the dithionite solution is relatively unstable, it is generally prepared just before use. Dry dithionite can be prepared by different methods and it is more stable than the solution, but flammable. However, it is more practical to prepare the dithionite in situ from liquid stable starting materials to avoid the handling of flammable or fluffy powder.
One method for preparing dithionite solution based on sodium borohydride is disclosed in U.S. Pat. No. 4,788,041, wherein the reaction equation is the following:[NaBH4+3.2NaOH]+4.8NaOH+8SO2→4Na2S2O4+NaBO2+6H2O
Generally [NaBH4+3.2NaOH] represents sodium borohydride solution containing about 12% NaBH4, about 40% NaOH and about 48% water. One example of such generally used commercially available solution is Borino™ (Finnish Chemicals Oy).
It is important to carry out the reaction in the right pH, because at too acidic range the yield is decreased because of the hydrolysis of borohydride and on the other hand at too high pH the yield of the main reaction is decreased. In U.S. Pat. No. 4,788,041 it is mentioned that the optimal pH is 5.5-6. According to said publication the hydrolysis of the borohydride can be decreased by lowering the reaction temperature to the range of 7-10° C.
In EP 1524241 it is disclosed that lye and a part of the sulfur dioxide can be introduced as ready sodium bisulfite solution, which has been prepared from sulfur-containing gases as follows:NaOH+SO2→NaHSO3 
If sodium bisulfite is used in the preparation of sodium dithionite, the lye of the sodium borohydride solution must be neutralized in order to obtain a pH low enough for the reaction. In US2004/0000380 and WO 88/10334 phosphoric acid is provided as a solution for this. As a disadvantage in these processes different kinds of phosphates are formed, which are not desired for example in the view of paper making. Furthermore, in the examples of the publication sulfuric acid is used, but then sulfate is obtained as a useless by-product. For example in paper machine environment sulfur causes corrosion and problems in the prevention of slime.
U.S. Pat. No. 5,562,803 discloses a multi-phase bleaching method of de-inked recycled pulp, wherein the pH is adjusted between the phases with acetic acid or other organic acids. Although it is not exactly preparation of dithionite, it is however formed in situ. The method is expensive and as a by-product organic compounds are formed, which among other things add biological oxygen consumption.
It is therefore necessary to further develop methods for preparing dithionite, in which methods no unwanted by-products are formed and wherein the good stability of reagents and reaction products is obtained. Further, it is desirable to develop economically advantageous methods.